This disclosure relates generally to an ion implanter, and more specifically to using a collimator magnet and a neutral filter magnet in combination with deceleration stages to improve the performance of an ion implanter.
Ion implantation is a standard technique for introducing conductivity-altering impurities into workpieces such as semiconductor wafers. In a conventional beamline ion implanter, such as a high current ion implanter, an ion source generates an ion beam and extraction electrodes extract the beam from the source. An analyzer magnet receives the ion beam after extraction and filters selected ion species from the beam. The ion beam passing through the analyzer magnet then enters a first deceleration stage comprising multiple electrodes with defined apertures that allow the ion beam to pass through. By applying different combinations of voltage potentials to the multiple electrodes, the first deceleration stage can manipulate ion energies. A corrector magnet shapes the ion beam generated from the first deceleration stage into the correct form for deposition onto the wafer. In particular, the corrector magnet receives a divergent ion beam and collimates the beam. In addition, the corrector magnet filters out any ions from the beam that may have been neutralized while traveling through the beamline. A second deceleration stage comprising a deceleration lens receives the ion beam from the corrector magnet and further manipulates the energy of the ion beam before it hits the wafer. As the beam hits the wafer, the ions in the beam penetrate the surface of the wafer coming to rest beneath the surface to form a region of desired conductivity, whose depth is determined by the energy of the ions.
As devices shrink, following Moore's Law, the required junction depths are continuously getting shallower and the required energy is being reduced. This presents a particular challenge to modern ion implanter design as low energy beams, particularly high current ones, are very difficult to transport without significant loss of beam current. There are generally three parameters that semiconductor manufacturers try to control in order to maintain good throughput and tight process control in high current ion implanters—beam current, energy contamination and uniformity both of ion beam current density and angle of implantation. If the beam current is low, this will reduce the throughput of the implanter for a given total dose. A reduction in throughput will result in extra costs for a semiconductor manufacturer because more time and money is needed to perform additional implantations to account for the reduction in throughput. Energy contamination occurs when there is a small fraction of the ion beam that is at a higher energy than desired. This small fraction of the ion beam will rapidly increase the depth of the p-n junction that is formed when creating an integrated circuit and lead to degraded performance of the electronic circuit. If the ion beam current density and angle of implantation are not uniform, there will be varying device properties across the face of the wafer. Varying device properties can compromise the yield or cause the semiconductor manufacturer to work harder at other process steps to open the yield window to take into account the variations in the implant step.
High current ion implanters typically have the capability to allow semiconductor manufacturers to individually control beam current, energy contamination and uniformity of both ion beam current density and angle of implantation, or at best, address any two out of the three parameters. Currently available high current ion implanters are unable to take into account all three of the parameters at one time. For example, a semiconductor manufacturer could perform an ion implantation with high beam current and low energy contamination, but this implantation would generally suffer a significant loss of uniformity of both ion beam current density and angle of implantation; the yield impact makes this an unattractive choice. Likewise, an ion implantation that is able to address beam current and uniformity, would likely suffer energy contamination problems. This choice is selected by some device manufacturers because some devices can be designed to allow energy contamination. Finally, an ion implant process that has both good uniformity and low energy contamination typically only produces low beam current and thus low throughput; this choice is selected by many device manufacturers. None of these approaches can control beam current, energy contamination and uniformity of ion beam current density and angle of implantation in one implantation without making any sacrifices in any of the three parameters.